R.
norvegicus a rather large member of the mouse family. On
average, these rats reach nearly 400 mm nose-to-tail, and weigh
140 to 500 g. Males are usually larger than females. In natural
populations, these rats are covered with coarse, brownish fur
(sometimes splotched with black or white hairs) on their dorsal
surface, which usually lightens to a gray or tan color nearing
the underside. Various strains of these rats bred in captivity
may be white, brown, or black. The ears and tail are bald. The
length of the tail is shorter than the length of the body. Molars
are lophodont and the dentary is 1/1-0/0-0/0-3/3. The ears of
Norway rats are typically shorter than those of related species.

Although
not technically a seasonal breeder, a mating increase occurs
in the warmer months of the year. An average female is capable
of giving birth approximately seven times per year. Around 18
hours after giving birth, females experience postpartum estrus,
and mate again. This reproductive function is responsible for
the huge birthrates, which can reach 60 young each year per
female. After a short gestation period of 22 to 24 days, the
litter of approximately 8 pups is born. The young are very small
and underdeveloped. It takes 14 to 17 days for the young's eyes
to open. Newborns weigh an average of 5 grams and are milk-fed
until weaning occurs at 3 to 4 weeks, and the young then leave
the nest.

Mostly
nocturnal or active at dusk, they go about digging burrows,
foraging for food, and preparing nests during these hours. Often,
these rats take up residence in areas near water. They are excellent
swimmers, and are often referred to as "water rats".

They
are excellent foragers. Using their sense of smell and touch,
they are able to survive quite easily given that there is a
steady supply of any type of food. In metropolitan areas, they
survive mainly on discarded human food, and anything else that
can be eaten without negative consequences. Because of their
foraging habits, they act as seed dispersers. Their burrows
also tend to aerate the soil.

In
the wild, Rattus norvegicus is preyed on by any number
of carnivorous mammals, birds, and reptiles. Humans also kill
very large numbers of these animals as pests. However, they
are also one of the most studied animals in science. The species
has provided us knowledge to the cures of diseases, workings
of the brain, and effects of various substances on the mammalian
body.

Original
distribution assumed to be SE Sibera, N China, and Hondo region
(islands of Honshu, Shikoku, and Kyushu) of Japan, but introduced
worldwide where it is more common in colder climates of higher
N and S latitudes; in warmer regions and tropics restricted
to habitats highly modified by humans (e.g. sewers, buildings,
wharves, breakwaters, ports, and larger cities). Considered
extinct in Norway.